The invention relates to a method for measuring the torque of an AC motor, including a three phase motor.
One known manner of measuring the torque of an AC motor is by means of a mechanical torque transducer coupled to the output shaft of the motor.
In another known torque measurement technique, the component of the input current to the motor which is in-phase with the input voltage is measured. The torque of the motor is determined from this in-phase current component by assuming a roughly linear relation; or by empirically determined correspondence to known torque values, which values may have been predetermined for example by means of a mechanical torque transducer as plotted against various values of in-phase input current. This method affords only a rough approximation. The method also requires input discrimination circuitry discriminating against input current which is out-of-phase relative to the input voltage, to thus insure that only in-phase current components are measured.
The present invention provides a method for measuring torque of an AC motor which addresses and solves the above noted and other problems. Torque is measured from the sensed parameters of input voltage and current to the motor. The current is sensed regardless of its phase relative to the voltage, i.e. without discrimination against out-of-phase components. In preferred form, a method is provided for measuring the instantaneous signature torque waveform of a three-phase AC motor. The method requires no advance empirical plotting of data or the like.